1. Field of the Invention
The present invention relates to a structure of a coil for an electric motor, and more specifically, it relates to a coil structure for an electric motor which can reduce the unevenness of an electric constant to improve output and can decrease the protrusion of a coil end in a rotating shaft direction of the electric motor.
2. Description of the Prior Art
FIG. 1 shows a partial coil structure on the edge surface of a stator of an electric motor in accordance with a conventional embodiment.
The electric motor regarding this conventional embodiment is a three-phase motor which is constituted of a stator 24, and a rotor 27 arranged so as to confront the stator 24. The stator 24 is composed of a stator core 24a provided with a predetermined number of slots 30 facing the rotor 27, and coils 11U, 11V and 11W of the respective phases inserted into the slots 30.
Each of the coils 11U, 11V and 11W has a bundle structure of a plurality of lead wires, and these coils are wound through the slots 30 corresponding to the respective coils. In this connection, of each coil 11U, 11V or 11W, a part (a coil end) which is wound, being protruded from the edge surface of the stator core 24a, i.e., the edge of each slot 30, will hereinafter be called a crossover coil between the slots.
In FIG. 1, the stator 24 and the rotor 27 are depicted in a straight line, but in fact, the stator 24 and the rotor 27 are cylindrical. In a cylindrical space defined inside the stator 24, the rotor 27 is arranged so as to confront the stator 24, as described above. On the surface of the rotor 27 which confronts the stator core 24a, permanent magnets 26 are mounted. Therefore, when a predetermined AC driving current is fed to the coils 11U, 11V and 11W, a three-phase alternating field is generated in the stator 24 to generate torque on the rotor 27.
Such a coil structure has a problem that the so-called space factor of the coil (a ratio of a volume occupied by the coil in the slot to the total slot volume) decreases inconveniently.
That is, the constitution where the coil comprising the bundle of a plurality of lead wires is wound through the slots permits the assembly of the electric motor with a suitable workability. However, the space factor of the coil in each slot is low, for example, 50%, and in consequence, the miniaturization of the electric motor has been restricted. Furthermore, when the coil is constituted by bundling the plurality of lead wires, the size of the crossover coil increases, and also for this reason, the miniaturization of the electric motor is difficult. Moreover, when the lead wires are wound by hand, there is a problem that the unevenness of inductance disadvantageously occurs.
In order to solve the above-mentioned problems, the same applicant as in the present application has suggested in Japanese Patent Application No. 1227/1993 that portions (coil sides) of the coil to be inserted into the slots and coil ends arranged on the edge surface of the stator core be made of different parts and that these parts constitute one coil.
This constitution will be described with reference to FIG. 2.
In FIG. 2, a coil 10 is constituted of a plurality of molded coil pieces 20d (coil sides) which are molded into a bar shape so as to be inserted into the respective slots 30, and a pair of curved crossover coils 13, 13' having the bar shape. The molded coil pieces 20d are laminated and received in the corresponding slots 30. In this case, a lamination direction is the radial direction of the electric motor.
The plurality of crossover rolls 13, 13' between the slots 30 are arranged so as to be mutually separated at a predetermined interval in the radial direction of the electric motor, and they are connected with the edge surfaces of the corresponding molded coil pieces 20d. A space between each pair of crossover coils 13 and 13' of one phase is such a space as to receive each of the crossover coils 13 and 13' of the other phase. The surfaces of the molded coil pieces 20d and the crossover coils 13, 13' are insulated except at contact surfaces between the molded coil pieces 20d and the crossover coils 13, 13'. In FIG. 2, the stator 24 and the rotor 27 are depicted in a straight line for simplification, as in FIG. 1, but in fact, they are cylindrical.
In the constitution in FIG. 2 described above, the space factor of the coil in the slot is improved, but since the crossover coils 13, 13' are molded into the bar shape, these crossover coils 13, 13' are heavy and have a large unevenness of size. This size unevenness leads to uneven inductance, which causes the performance of the electric motor to deteriorate.
Furthermore, the surfaces of the crossover coils 13, 13' are very thinly insulated except at the contact surfaces with the molded coil pieces 20d, and thus an insulation reliability on the crossover portions between the slots 30 is low.